1. Field of the Invention
The present invention relates to a pneumatic booster for use, for example, in an automobile to reduce the brake pedal pressure required for braking.
2. Description of the Prior Art
FIGS. 1 to 4 show a tandem type pneumatic booster as one example of the above-described pneumatic booster.
The tandem type pneumatic booster has a housing 3 which comprises a front shell 1 and a rear shell 2. The interior of the housing 3 is divided by a center shell 4 into a pair of front and rear chambers 5 and 6, in which a front diaphragm 7 and a rear diaphragm 8 are disposed, respectively. The inner peripheral portion of the center shell 4 is slidably fitted with a valve body A. The inner peripheral portions of the front and rear diaphragms 7 and 8 are fixed to the valve body A. The outer periphery of the rear shell 2 is formed with a bead retaining portion 9. More specifically, as is most clearly shown in FIG. 2, the outer peripheral portion of the rear shell 2 is first bent rearwardly at a position 2b and then bent upwardly to form a U-shaped bead retaining groove 9a which opens radially outward. The U-shaped bead retaining groove 9a thus has side wall portions that generally oppose each other, and a bottom portion. The outer peripheral portion of the rear shell 2 that is bent in this way has an outer peripheral end portion 2a which extends radially outward beyond the bending position 2b. The bead retaining portion 9 is formed with an insertion-side edge portion 9b, which is passed by a bead portion of the diaphragm 8 (described later) when inserted into the retaining groove 9a.
The outer peripheral end portion 8a of the rear diaphragm 8 is formed with a ring-shaped bead 10. The bead 10 is accommodated in the groove 9a and clamped by the bead retaining portion 9 and the end portions of the center and front shells 4 and 1, thereby maintaining an airtight state in the rear chamber 6. The dimensions of bead 10 and the bead retaining portion 9 are set so that the inner diameter d.sub.i of the bead 10 is smaller than the outer diameter D.sub.o of the insertion-side edge portion 9b of the bead retaining groove 9a in the bead retaining portion 9, as shown in FIG. 2.
An input shaft 11 is connected to a brake pedal (not shown), and an output shaft 12 is brought into contact with a piston (not shown) of a master cylinder, thereby boosting the force that acts on the piston, and thus enabling a reduction in the pedal pressure required for braking.
In this tandem type pneumatic booster, the inner diameter d.sub.i of the bead 10 is smaller than the outer diameter D.sub.o of the insertion-side edge portion 9b of the bead retaining portion 9. Accordingly, when the diaphragm 8 is to be attached to the rear shell 2, if the rear shell 2 is merely pressed in the direction of the arrow F, with the rear diaphragm 8 being attached to the inner side of the center shell 4, as shown in FIG. 3, the insertion-side edge portion 9b of the bead retaining portion 9 abuts against the bead 10, and it is therefore impossible to assemble these members. For this reason, the bead 10 is bent in advance so as to be located outside the rear end portion 4a of the center shell 4, as shown in FIG. 4, and with the bead 10 left in this bent position, the rear shell 2 is pressed in the direction of the arrow F. After the rear shell 2 has been forced into the center shell 4 to an appropriate position, the bent bead 10 is rebent to fit into the groove 9a. Then, the rear shell 2 is further pressed in the direction of the arrow F until the rear end portion 4b of the center shell 4 abuts against the outer peripheral end portion 2a of the rear shell 2. In this way, the center shell 4, the rear shell 2 and the rear diaphragm 8 are assembled together in an integral structure. The bead 10 of the diaphragm 8 is retained within the groove 9a by the rear end portion 4a of the center shell 4, and there is therefore no fear of the bead 10 disengaging from the groove 9a. Thereafter, the front shell 1 is attached to complete the assembly, as shown in FIG. 1.
However, the conventional pneumatic booster of the type described above needs to bend the bead 10, as shown in FIG. 4, when it is assembled, and hence additional time and labor are required to perform the bending operation, resulting in a lowering in assembly efficiency. In addition, there is a danger that the edge of the bead retaining portion 9 will abut and damage a portion of the diaphragm 8 other than the bead 10 during the assembly process. Accordingly, care must be taken not to damage the diaphragm 8, and this causes assembly efficiency to be further impaired.